The recharge of a starter battery normally is effected with the aid of an AC generator. One such prior-art generator is supplied with regulating voltage from a voltage regulator which in turn senses a reference voltage from the positive terminal of the battery. If the reference voltage is lower than a value corresponding to a certain nominal charging voltage, the regulator will affect the generator, bringing the latter to increase the charging voltage supplied to the battery. In a corresponding manner, the generator will reduce the charging voltage supplied to the battery when the reference voltage exceeds the nominal reference voltage. This arrangement ensures that recharging of the battery always is effected at a voltage value that is essentially constant and equal to the nominal charging voltage.
It is generally desired that the battery may be charged as quickly as possible in order that the battery be ready quickly to cope with new current drains. In principle, this possibility is conditioned on the nominal value of the voltage being set to a comparatively high level to ensure that the charging of the battery progresses at a high level of charging current for as long as possible. In reality, however, it is necessary to restrict the nominal voltage, on the one hand not to risk damages on the battery, and on the other to avoid gassing in the battery, which would involve safety hazards.
From FR 2 550 396 is previously known a device by means of which a battery may be charged rapidly at a voltage not exceding the nominal voltage. This is achieved by introducing a certain voltage drop between the positive terminal of the battery and the regulator, whereby the regulator is "conned" into acting as if the reference voltage is lower than it actually is. In this manner the voltage delivered by the generator will amount to the nominal voltage increased by the introduced voltage drop. In order not to risk that the battery be damaged, this voltage drop is maintained over a predetermined, restricted period of time, say over 30 minutes, after which period the charging again equals the nominal charging voltage.
One disadvantage inherent in this method is that it impossible to establish whether or not the battery has succeeded in reaching the fully charged condition after the lapse of the selected period. In accordance with one embodiment of this prior-art device the voltage drop therefore may be introduced in response to the magnitude of the battery charging current, which is measured and is compared with a predetermined limit value. As long as the charging current does not exceed this limit value, the voltage drop will be applied, and the battery will be charged at a voltage that is higher than the nominal voltage. This version, too, suffers from a disadvantage inasmuch as the measurement of the current is effected by means of a Hall probe with resulting too complicated and too expensive a measuring procedure.